The invention relates to a light-curable, negative-working mixture and to a recording material which is obtainable therefrom and which is suitable for producing elastic relief printing plates.
Light-curable elastomeric mixtures are known and usually contain an elastomeric binder, a compound polymerizable by a free-radical mechanism and a polymerization initiator which can be activated by actinic radiation. Printing plates having a light-sensitive layer composed of such a mixture are described, for example, in DE-A 2,215,090 (=GB-A 1,366,769). However, they can be developed after exposure, to give the relief image, only by means of organic solvents.
In Patent Publications DE-A 3,003,011 (=ZA 80/506), EP-A 0,130,828, DE-A 2,725,730 (=U.S. Pat. No. 4,042,386) and DE-A 3,322,994 (=U.S. Pat. No. 4,554,240), recording materials which can be developed in an aqueous medium and which contain water soluble polymers as binders, in particular partially or completely saponified polyvinyl acetates, are admittedly disclosed. However, the water resistance of the relief printing plates produced from them is low, so that they cannot be used with water-dilutable printing inks.
The light-sensitive mixtures according to DE-A 3,012,841 (=U.S. Pat. No. 4,272,608) contain, as binders, carboxylated butadiene/acrylonitrile copolymers in which at least 25% of the carboxyl groups are neutralized with a metal cation from groups IIA or IIB of the periodic table.
Butadiene/styrene, isoprene/styrene and butadiene/acrylonitrile copolymers modified by reaction with 3-mercaptopropionic acid and thus containing carboxyl groups, and similarly modified polybutadienes and polyisoprenes, are disclosed in U.S. Pat. No. 4,446,220 as binders in light-sensitive compositions.
Rubber-elastic copolymers, which contain carboxyl groups and are formed by copolymerization of 1,3-butadiene with (meth)acrylic acid, an ethylenically polyunsaturated compound, such as ethylene glycol dimethacrylate or divinylbenzene, and an ethylenically monounsaturated compound, such as ethyl acrylate, are described in EP-A 0,277 418 and 0,261,910. Said copolymers are used as binders in light-sensitive compositions.
A light-sensitive elastomeric composition, which contains a high molecular-weight carboxylated butadiene/acrylonitrile copolymer as binder, for flexographic printing planes is described in U.S. Pat. No. 4,517,279.
The composition disclosed in GB-A 2,179,360 contains, in addition to the carboxylated butadiene/acrylonitrile copolymer, also a chlorosulfonated polyethylene, and it can likewise be developed with an aqueous developer.
According to EP-A 0,183,552, 1,3-butadiene/methacrylic acid, 1,3-butadiene/methacrylic acid/acrylonitrile or butadiene/methacrylic acid/styrene copolymers are used as binders which contain carboxyl groups.
The light-sensitive composition according to EP-A 0,251,228 contains a rubber elastic binder having a glass temperature lower than -0.degree. C. Copolymers containing carboxyl groups, such as 1,3-butadiene/(meth)acrylic acid/acrylate copolymers are also mentioned as examples of such a binder. The flexographic printing plates produced using this composition are developable in water.
In EP-A 0,273,393, chlorine-containing polymers having a glass temperature lower than +5.degree. C. are described as binders in light-sensitive compositions for flexographic printing plates.
If the carboxyl group content in the polymeric binders is too low, the flexographic layer can be developed with aqueous developers only with difficulty. In these cases, it is necessary to add to the light-sensitive composition polar monomers or auxiliaries in a large amount, which in turn entail a high cold flow of the unexposed material. "Cold flow" is used as the description of plastic flow over a prolonged period due to the intrinsic weight of the material, internal strains in the material or external stresses. Although an increase in the carboxyl group content results in an improved aqueous developability, the rubber-elastic properties deteriorate at the same time and, furthermore, the resistance to water-containing printing inks is reduced.